Method and apparatus for rapid imprint lithography

ABSTRACT

In accordance with the invention, a mold for imprinting a patterned region by imprint lithography is provided with a peripheral groove around the patterned region. The groove is connected, as by channels through the mold, to a switchable source for gas removal to prevent bubbles and for the application of pressurized gas to separate the mold and substrate. 
     In use, the mold is disposed adjacent the moldable surface and gas is withdrawn from the patterned region through the groove as the mold is pressed toward and into the moldable surface. At or near the end of the imprinting, the process is switched from removal of gas to the application of pressurized gas. The pressurized gas passes through the groove and separates or facilitates separation of the mold and the moldable surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/867,515 of the same title filed by Wei Zhang on Nov. 28, 2006 andwhich is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for performing imprintlithography. It is particularly useful for providing high resolutionmicroscale or nanoscale imprint lithography at high speed.

BACKGROUND OF THE INVENTION

Lithography is a key process in the fabrication of semiconductorintegrated circuits and many optical, magnetic, biological andmicromechanical devices. Lithography creates a pattern on a thin filmcarried on a substrate so that, in subsequent process steps, the patterncan be replicated in the substrate or in another material that is addedonto the substrate.

Conventional lithography, referred to as photolithography, involvesapplying a thin film of photosensitive resist to a substrate, exposingthe resist to a desired pattern of radiation and developing the exposedresist to produce a physical pattern on the substrate. Unfortunately,the resolution of patterns produced by photolithography is limited bythe wavelength of the exposing radiation. Moreover, as pattern featuresbecome smaller, increasingly expensive shorter wavelength equipment isrequired.

Imprint lithography, based on a fundamentally different principle,offers high resolution, high throughput, low cost and the potential oflarge area coverage. In imprint lithography, a mold with a pattern ofprojecting and recessed features is pressed into a substrate-supportedmoldable surface such as a thin film of polymer, deforming the shape ofthe film to form a relief pattern in the film. After the mold isremoved, the thin film can be processed, as by removing reducedthickness portions of the film. Such removal exposes the underlyingsubstrate for further processing such as etching, doping, or deposition.Imprint lithography can be used to replicate patterns having highresolution features in the microscale and nanoscale ranges. Details ofnanoscale imprint lithography (“nanoimprint lithography”) are described,for example, in U.S. Pat. No. 5,772,905 issued Jun. 30, 1998 andentitled “Nanoimprint Lithography”. The '905 patent is incorporatedherein by reference.

A potential limitation on the rate of high speed manufacturing usingimprint lithography is the presence of gas between the mold and themoldable film. Pressing the mold too rapidly can entrap gas bubbles intiny recessed regions, deteriorating the resolution of the imprintedpattern. A second limitation is the separation of the mold and theimprinted substrate. Typically, after pressing, the mold and substrateare mechanically separated from the edge by inserting a wedge betweenthe mold and substrate. This separation from the edge usually requiresthat the mold and substrate be transported from the site of the pressingapparatus to the site of the separation apparatus. The separation stepthus limits throughput of imprinting. Furthermore, this conventionalseparation may cause cracking at the edge of the mold or substrate. Itthus, contributes to mold wear, increases operating cost, and limitsthroughput.

Accordingly, it would be highly desirable to provide methods andapparatus to permit more rapid pressing and separation in imprintlithography.

BRIEF SUMMARY OF THE INVENTION

In accordance with the invention, a mold for imprinting a patternedregion by imprint lithography is provided with a peripheral groovesubstantially around the patterned region. The groove is connected, asby channels through the mold, to a switchable fluid source for theapplication of pressurized fluid to separate the mold and substrate.Advantageously, the same grooves and channels may be used for gasremoval to prevent bubbles.

In preferred use, the mold is disposed adjacent the moldable surface andgas is withdrawn from the patterned region through the groove and thechannels as the mold is pressed toward and into the moldable surface.After the mold contacts the moldable surface, gas removal is eithercontinued or stopped. At or near the end of the imprinting, the processis switched from removal of gas to the application of pressurized fluidsuch as gas. The pressurized fluid passes through the channels and thegroove, and it separates or facilitates separation of the mold and themoldable surface.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The advantages, nature and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection with theaccompanying drawings. In the drawings:

FIG. 1 is a flow diagram of the steps involved in imprinting a patternin accordance with the invention;

FIGS. 2 and 3 are schematic cross sectional and elevational views of amold useful in the process of FIG. 1;

FIG. 4 schematically illustrates the molding apparatus at various stagesof the process of FIG. 1; and

FIGS. 5, 6 and 7 depict apparatus and results of an experimentaldemonstration of the invention.

It is to be understood that the drawings are for purposes ofillustrating the concepts of the invention and are not to scale.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 is a flow diagram of the stepsinvolved in imprinting a pattern in accordance with the invention. Thefirst step shown in block A is to provide a substrate having a moldablesurface and a grooved mold having a patterned region of projecting andrecessed features for imprinting a desired pattern.

The substrate having a moldable surface typically comprises a substratecoated with a thin moldable layer such as a polymer resist. Thesubstrate can be any one of a wide variety of materials such assemiconductors, polymers, dielectrics, and conductors. The moldablelayers can be coatings of polymers or layers of powdered materials. Aparticularly advantageous coated substrate is crystalline silicon orsilicon dioxide coated with a thermal or UV curable polymer. Exemplarycombinations of substrate and resist are set forth in the aforementioned'905 patent incorporated herein by reference.

An exemplary grooved mold is schematically illustrated in FIG. 2 (crosssectional view) and FIG. 3 (elevational view). Referring to FIG. 2, themold 200 typically comprises a substantially planar surface thatincludes at least one pattern region 200A to replicate a pattern byimprinting. The pattern region is composed of projecting features 201Aand recessed feature 201B that, upon pressing into a moldable surface,imprint a pattern that has recessed features corresponding to moldprojecting features 201A and projecting features that correspond to moldrecessed features 201B. In highly advantageous applications, the moldhas a pattern of projecting and recessed features for imprinting atleast one feature having a minimum lateral dimension of less than 200nanometers.

The mold surface is provided with a grooved trench 202 on the moldsurface that includes pattern region 200A. The groove 202 is adjacentthe pattern region 200A and preferably substantially surrounds thepattern region. The groove 202 is in physical communication with one ormore through-holes 203 in the mold so that fluid (e.g. gas) can bepumped from the groove 202 via the through-hole(s) 203 from themold/substrate interface. Alternatively, the through-holes could bereplaced by additional surface channels to edges of the mold or in-bodychannels to sidewalls of the mold (not shown). Such pumping minimizesentrapped gas and facilitates molding. In addition fluid can be pumpedinto the groove 202 after or near the end of imprinting to facilitaterelease of the mold from the substrate. The fluid can be gas or liquid.Typically it is air or inert gas.

The shape of the groove 202 around the pattern region is advantageouslyin conformation with the outer periphery of the pattern region 200A sothat the groove 202 is closely adjacent the boundary of the patternregion. Preferably the minimum distance between the groove and thepattern boundary is about 0.5 millimeter to 2.0 millimeter. Typicalgroove shapes are circular, or polygonal (e.g. rectangular) to surroundindividual pattern regions. However the groove shape can be zigzag topartially surround each of a plurality of pattern regions. The depth ofthe groove 202 is preferably in the range from about 1.0 micrometer toabout 2.0 millimeters, although groove depth can range from about 50nanometers to about one centimeter depending on the thickness of themold. The groove can have any of a wide variety of cross sectionalshapes. Typical cross sections can be squares, rectangles, triangles,semi-circles and parallelograms. The through-holes can be any desiredcross section, but circular cross sections are typical. The grooveeffective diameters are typically in the range from about 5 micrometersto about 10 millimeters. At least one through-hole connected to thegroove is preferred. Multiple through-holes connected to the groove areeven more preferable for even flow of gas or fluid.

The mold 200 is advantageously fabricated of quartz, fused silica, metalsemiconductor, polymer or a combination of these materials. The moldbody thickness should provide sufficient mechanical strength for moldingafter the grooved trench and through-holes are made. Typically, bodythickness is about 2 millimeters or greater. The pattern region 200A canbe made by any of a wide variety of techniques depending on minimumfeature size. For patterns that include nanoscale minimum features, thepattern region can be made by electron beam lithography. The groovetrench and through-holes can be made by conventional techniquesincluding machining (mechanical, laser, ultrasonic or jet), lithographicpatterning and etching (wet chemical or dry plasma) or a combination ofthese techniques. The mold can be a homogenous body or multilayer ofvarious materials. Also, the mold can be made by bonding several bodiestogether.

Referring to Block B of FIG. 1, after the mold and substrate areprovided, the next step is to dispose the mold adjacent the substrate.

FIG. 4A illustrates the mold 400 disposed adjacent the substrate 401with the pattern 405 in position for imprinting the moldable surface402. The mold is positioned so that the groove 403 is adjacent themoldable surface of substrate 401. The mold and substrate can bedisposed in air or in a low pressure gas ambient.

The third step (referring to Block C of FIG. 1) is to press together themolding surface of the mold and the moldable surface of the substrateand, during at least a portion of the pressing, to remove gas throughthe groove and the through-holes.

FIG. 4B shows the mold 400 and substrate 401 pressed together with thepattern region of projecting and recessed features pressing against themoldable surface. The arrows pointing outward from through-holes 404indicate gas removal from the mold/substrate interface through thegroove 403 and the through-holes 404. While some gas will be passivelyremoved by the reduction in volume as the mold and substrate are pressedtogether, it is preferred to actively pump gas out through the grooveand through-holes. Active removal is effected by attaching thethrough-holes to a low-pressure reservoir (not shown) or a gas pump (notshown). Gas will flow from the region between the mold and the substrateto a lower pressure region connected to the through-holes.

Gas removal can be initiated prior to the start of the pressing step orafter the start of pressing. The gas removal should begin early enoughto remove gas between the pattern region and the moldable layer beforethe pattern region seals against the moldable layer.

The pressing can be affected in a variety of ways. One approach is topress the mold and the substrate together by a high precision mechanicalpress. For further details see U.S. Pat. No. 5,772,905.

Another approach is pressing by fluid pressure. The interface betweenthe mold and the substrate is sealed from pressurized fluid, such ascompressed air or inert gas that presses the mold and substratetogether. For further details see U.S. Pat. No. 6,482,742, which isincorporated herein by reference. In this mode of pressing, thewithdrawal of gas prior to pressing can not only assist in preventingbubbles but also can remove ambient gas diffused into interface regionto increase the pressure difference between the interface region and theambient. The withdrawal of gas creates a pressure difference between theinterface region and the ambient. When the ambient pressure isincreased, the pressure difference is increased sufficiently high toseal the interface. The withdrawal, thus, helps to seal the interface.Pressurized fluid can be introduced around the sealed mold/substrateassembly to imprint the patterned region into the moldable surface. Theprocess can achieve sealing without using a flexible film to seal theinterface, and, can be automatically sequenced for rapid manufacturing.Other approaches to pressing include pressing driven by the applicationof electrostatic or magnetic force.

The next step is to permit the moldable surface to harden sufficientlyto retain the imprinted pattern (referring to Block D of FIG. 1). Duringthe imprinting, the molding surface is sufficiently sealed against themoldable surface that no appreciable additional gas is withdrawn. Thiscondition is illustrated in FIG. 4C. The moldable surface, typicallyresist, fully fills the space between the mold and the substrate. Theresist is hardened, as by UV or thermal curing, or cooling athermoplastic below its plastic transition temperature. The substrateand the mold may be held together by the hardened resist. The imprintingpressure can be removed before hardening or be maintained duringhardening.

The fifth step (referring to Block E of FIG. 1) is to separate the moldfrom the substrate. This separation can be achieved or assisted by theapplication of pressurized fluid through the through-holes and thegroove into the mold/substrate interface. The inwardly directed arrowsin FIG. 4D represent pressurized fluid passing into the through-holesand the groove to the mold/substrate interface. The fluid here can begas or liquid. The pressurized fluid first builds up at the grooveregion, and then at the interface region to separate the mold from themolded surface on the substrate.

The substrate and mold can then be moved apart. An advantageous optionalenhancement of the separation step is to provide the substrate with asupport having one or more vacuum attachment regions 40B. The evacuationof regions 40B through vacuum grooves (not shown) on body 403 can securethe substrate when the substrate is lifted off as shown in FIG. 4E. Theinjection of pressurized fluid into the interface and the application ofvacuum to retain the substrate can be automatically sequenced. Thus eachstep in the imprinting process can be automatically controlled andsequenced for high speed manufacture. A mold that has a blank (smooth,flat) molding surface without surface replication features can be usedto planarize a polymer layer to avoid unwanted surface variation of thesubstrate.

The invention may now be more clearly understood by consideration of thefollowing experiment performed to demonstrate the workability of themethod. FIG. 5 shows an enlarged view of the mold used for theexperiment. The body 500 of the mold is approximately 1.5 inch×1.5 inch,0.25 inch thick and is made of quartz. The mold includes a circulargrooved trench 501 and two through-holes 502. The grooved trench hasouter diameter of about 25 millimeters, a trench width of about 1millimeter and a trench depth of about 1 millimeter. The twothrough-holes 502 directly connect to the grooved trench 501. Themolding surface has surface replication features 503 in sizes rangingfrom several micrometers to several millimeters. The surface replication2

FIG. 6 a is a photo showing the testing setup with which the experimentwas performed. The testing setup consists of gas/vacuum control 601, abase plate 602, a top plate 603 and a user interface computer controlpanel (not shown). The computer control panel controls the supplies ofgas and vacuum pumping. However, the process tested was controlledmainly by the manual valves 601.

FIG. 6 b shows the mold 604 and bottom plate 605 of the testing setup.The vacuum grooves to hold the mold against the bottom plate can be seenthrough body of the mold. There are two holes 606 through the bottomplate surrounded by two circular grooves respectively. The two holes onthe bottom plate are the two through-holes.

Referring to FIG. 6 c, the substrate 608 used in the experiment was puton top of the mold with surface to be imprinted contacting with topsurface of the mold. In a commercial embodiment, this placement could bedone by a conventional positioner. A UV curable resist layer was coatedon the surface of the substrate. The top plate with O-ring 607 is alsoshown at the right of the photo. When the top plate was installed on thetop of the mold, the recessed area of the top plate formed a sealedchamber. The substrate was cut from a Silicon wafer to fit into thechamber and to cover the grooved trench of the mold.

FIG. 6 d depicts the installed setup ready for testing. Connector 609goes to the sealed chamber. Connectors 610 go to the through-holes ofthe bottom of the plate, then to the through-holes of the mold andfinally to the grooved trench of the mold. Connector 611 goes to thevacuum grooves on the surface of the bottom plate that holds the moldagainst the bottom plate. Screws hold the sealed chamber for vacuumingand pressurizing.

The experimental process was as follows: (1) load the mold on the bottomplate and turn on the vacuum to hold the mold; (2) coat the UV curablelayer on top of the substrate; (3) load the coated substrate on themold; (4) install the top plate on top of the mold and clamp the sealedchamber with screws; (5) turn on vacuuming inside the chamber andvacuuming through the grooved trench; (6) turn off the vacuuming insidethe chamber and maintain vacuuming through the grooved trench; (7) fillpressurized nitrogen into the chamber; (8) hold for a time period; (9)cure the UV curable layer by UV exposure through the transparent body ofthe mold; (10) release the pressure inside the chamber and take off thetop plate. The vacuuming through the grooved trench could be turned offanytime after step 7; (11) flow pressurized nitrogen through the groovedtrench; (12) separate the substrate from the mold by the blowing gas.

In ten runs the substrates were separated from the mold with the inletpressure of blowing nitrogen set at or gradually increased toapproximate 30 psi. The surface replication features enclosed by thegrooved trench were replicated into the UV cured layer on all runs.

FIG. 7 shows a typical result of imprinting from the experiment. Image700 is the substrate with the trace of the grooved trench 701 andsurface patterns replicated from the mold. The trace of the groovedtrench is a closed loop indicating a good seal for the imprint. Thepatterns were replicated uniformly although variation of colors around aparticle on the substrate was seen. Image 702 shows imprinted patternson the substrate near the edges of the grooved trench 704, 705. On theimage, patterns with good imprinting quality were obtained right to theinner edge of the grooved trench 704. Image 703 shows the imprintedpattern close to the center of the substrate. The imprinting quality ofthe patterns is very good.

It can now be seen that one aspect of the invention is a method ofimprinting a substrate having a moldable surface. It comprises providingthe substrate and providing a mold having a molding surface forimprinting onto the moldable surface. The molding surface issubstantially surrounded by a surface groove for conducting fluid. Themold is disposed adjacent the substrate with molding surface adjacentthe moldable surface. The molding surface is pressed against themoldable surface to imprint the moldable surface. Prior to and/or duringthe pressing, gas is advantageously withdrawn through the surfacegroove. The moldable surface is hardened to retain the imprint, andpressurized fluid is then applied through the groove to facilitateseparation of the mold from the imprinted moldable surface.

Another aspect of the invention is apparatus for imprinting a moldingsurface on a substrate having a moldable surface. The apparatusincluding, in operative relation, a mold having a molding surface thatis substantially surrounded by a surface groove for conducting fluid. Apositioner can be provided to dispose the mold adjacent the substratewith the molding surface adjacent the moldable surface.

The apparatus can include a pump or low-pressure gas reservoirswitchably connected to withdraw gas through the surface groove. Itincludes a source of pressurized fluid switchably connected to applypressurized fluid through the surface groove.

Apparatus is provided to press the mold against the substrate, and acontroller is provided to direct the introduction of pressurized fluidto separate the mold and the substrate. The controller mayadvantageously also direct the withdrawal of gas before and/or duringpressing.

It is to be understood that the above-described embodiments areillustrative of only a few of the many possible specific embodimentswhich can represent applications of the invention. Numerous and variedother arrangements can be made by those skilled in the art withoutdeparting from the spirit and scope of the invention.

1. A method of imprint lithography comprising the steps of: providing asubstrate having a moldable surface; providing a mold having a moldingsurface for imprinting onto the moldable surface, the molding surfacesubstantially surrounded by a surface groove for conducting fluid;disposing the mold adjacent the substrate with the molding surfaceadjacent the moldable surface; pressing the molding surface against themoldable surface to imprint the moldable surface; hardening the moldablesurface; and applying pressurized fluid through the groove to facilitateseparation of the mold from the imprinted moldable surface.
 2. Themethod of claim 1 further comprising the step of withdrawing fluidthrough the surface groove prior to or during the pressing.
 3. Themethod of claim 1 wherein the pressing comprises sealing the regionbetween the molding surface and the moldable surface to produce a sealedmold/substrate assembly and subjecting the mold/substrate assembly todirect fluid pressure.
 4. The method of claim 3 wherein the sealingcomprises withdrawing fluid through the surface groove.
 5. The method ofclaim 1 wherein the pressing comprises pressing by a mechanical press.6. The method of claim 1 further comprising a step of moving apart saidsubstrate and said mold.
 7. The method of claim 1 wherein said moldcomprises a plurality of voids to connect said surface groove at one endand the surface opposite to said molding surface or the sidewall surfaceat the other end.
 8. The method of claim 7 wherein said voids providefluid passage to said surface groove.
 9. The method of claim 1 whereinsaid surface groove is made by either machining, or by a lithographicalpatterning and etching.
 10. The method of claim 1 wherein said moldcomprises a plurality of bodies bonded together.
 11. Apparatus forimprinting a molding surface on a substrate having a moldable surfacecomprising in operative relationship: a mold having a molding surface,the molding surface substantially surrounded by a surface groove forconducting fluid; a positioner for placing the mold adjacent thesubstrate with the molding surface adjacent the moldable surface;pressing apparatus for pressing the mold against the substrate; and asource of pressurized fluid switchably connected to introducepressurized fluid through the surface groove to facilitate separation ofthe mold from the imprinted substrate.
 12. The apparatus of claim 11further comprising a controller to control the sequence of imprintingthe moldable surface and introducing pressurized fluid into the grooveto facilitate separation of the mold from the imprinted moldablesurface.
 13. The apparatus of claim 11 further comprising a pump or alow pressure reservoir switchably connected to the surface groove towithdraw fluid through the groove.
 14. The apparatus of claim 13 furthercomprises a controller to control the sequence of withdrawing gasthrough the groove, imprinting the moldable surface and introducingpressurized fluid to facilitate separation of the mold from theimprinted surface.
 15. The apparatus of claim 11 wherein the moldfurther comprises a plurality of through-holes connected to the surfacegroove.
 16. The apparatus of claim 11 wherein the mold comprises apattern of projecting and recessed features for imprinting at least onefeature having a minimum lateral dimension of less than 200 nanometers.17. The apparatus of claim 11 wherein the pressing apparatus comprises amechanical press.
 18. The apparatus of claim 11 wherein the pressingapparatus comprises a pressure vessel for enclosing the mold andsubstrate and a source of pressurized fluid for introducing pressurizedfluid into the chamber.
 19. The apparatus of claim 18 further comprisinga controller to control the sequence of introducing pressurized fluidinto said vessel to imprint the moldable surface and introducingpressurized fluid into the groove to facilitate separation of the moldfrom the imprinted moldable surface.
 20. The apparatus of claim 18further comprising a pump or a low pressure fluid reservoir switchablyconnected to the surface groove to withdraw fluid through the groove.21. The apparatus of claim 20 further comprising a controller to controlthe sequence of disposing the mold and substrate in the pressurechamber, withdrawing fluid through the groove to seal the region betweenthe molding surface and the moldable surface, introducing pressurizedfluid into the chamber to imprint the moldable surface, and introducingpressurized fluid into the surface groove to facilitate separation ofthe mold and the imprinted substrate.
 22. The apparatus of claim 11wherein said mold comprises a plurality of voids to connect said surfacegroove at one end with the surface opposite to said molding surface orthe sidewall surface at the other end.
 23. The apparatus of claim 22wherein said voids provide fluid passage to said surface groove.
 24. Theapparatus of claim 11 wherein said surface groove is made by eithermachining or by lithographic patterning and etching.
 25. The apparatusof claim 11 wherein said mold comprises a plurality of bodies bondedtogether.